Thermally most stable oxide is

 $\mathrm{Mg}$ is an alkaline earth metal element. It has high hydration energy due to its small ionic radii. $\mathrm{Mg}$ is inert to water and oxygen because of formation of oxide layer on their surface. $\mathrm{MgO}$ is a most stable oxide thermally. $\mathrm{MgO}$ is a refractory material that is chemically and physically stable at high temperature. It has high thermal conductivity and low electrical conductivity.

Therefore, option B is correct.